Switchgear
From Wikipedia, the free encyclopedia
The term switchgear, commonly used in association with the electric power system, or grid, refers to the combination of electrical disconnects and/or circuit breakers used to isolate electrical equipment. Switchgear is used both to de-energize equipment to allow work to be done and to clear faults downstream.
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[edit] Locations
Switchgear is located anywhere that isolation and protection may be required. These locations include generators, motors, transformers, substations, and high or medium voltage distribution networks. Switchgear of all types may be used by the power utility as protection against line to ground, phase to phase, or line to neutral faults. Switchgear is also used for heavy industry purposes. High or medium voltage equipment may be required for industrial processes and switchgear is used in these locations to isolate and protect this equipment.
[edit] Substations
Typically switchgear in substations is located on both the high voltage and the low voltage side of large power transformers. The switchgear located on the low voltage side of the transformers in distribution type substations, now are typically located in what is called a Power Distribution Center (PDC). Inside this building are typically smaller, medium-voltage (~15kV) circuit breakers feeding the distribution system. Also contained inside these Power Control Centers are various relays, meters, and other communication equipment allowing for intelligent control of the substation.
[edit] Housing
Switchgear for low voltages may be entirely enclosed within a building. For transmission levels of voltage (high voltages over 66 kV), often switchgear will be mounted outdoors and insulated by air, though this requires a large amount of space.
At small substations, switches may be manually operated, but at important switching stations on the transmission network all devices have motor operators to allow for remote control.
[edit] Types
A piece of switchgear may be a simple open air circuit breaker or it may be insulated by some other substance. An effective although more costly form of switchgear is "gas insulated switchgear" (GIS), where the conductors and circuit breakers are insulated by pressurized sulfur hexafluoride gas. Another common type is oil insulated switchgear. Vacuum circuit breakers are frequently used as medium voltage switchgear (up to 35 kV).
[edit] Functions
One of the main basic functions of switchgear is protection: discrimination between circuit breakers enhances availability, that is to say continuity of service. The overall approach is termed coordination: the standards provide a framework for discrimination and cascading that protects the integrity of the power system and minimizes the scope of downstream outages.
